Talking about on-site debugging of frequency converter

With the continuous advancement of science and technology, frequency converters are constantly updated, replaced, upgraded, and evolved, and their applications are becoming more and more extensive. In particular, the excellent speed regulation, energy saving, soft start and other performances of frequency converters are favored by the majority of users. As a user of frequency converters, it is very important to master and be familiar with the correct on-site debugging methods and technical essentials of frequency converters, which is crucial for the normal operation of frequency converters, reduce failures, and extend service life. Based on many years of practical work experience in frequency converter theoretical research and actual application, the author provides the following debugging experience for communication with colleagues in the hope of inspiring others.

2 Main steps of debugging

There are five main steps to debug the inverter.

2.1 Frequency Converter System Function Investigation

Before debugging the inverter, you must carefully read the instruction manual and related information of the inverter to be debugged, and be familiar with its use environment and precautions. Especially before the inverter is powered on, you must carefully observe and check whether the inverter has obvious signs of failure, such as whether its input and output terminals meet the requirements of the manual. Pay special attention to whether there are any new contents added.

2.2 Inverter no-load operation debugging

The following three steps are the most basic and most important debugging operations for inverter no-machine (i.e. not connected to the motor) and no-load (i.e. the motor is not loaded) operation debugging:

(1) Ground the inverter's grounding terminal and connect its power input terminal to the power supply through a leakage protection switch;

(2) Check whether the factory display on the inverter display window is normal. If it is incorrect, reset it, otherwise ask the supplier to return it;

(3) Be familiar with the inverter’s operating keys.

Generally, frequency converters have six keys: run, stop, prog, data/enter, up▲, down▼. The definitions of the operation keys of different frequency converters are basically the same. In addition, some frequency converters also have function keys such as monitor/display, reset, jog, and shift. These keys need to be debugged.

2.3 Inverter no-load operation debugging

The following four steps are crucial for debugging the inverter with machine (i.e. connected to the motor) or no-load (i.e. the motor is not loaded).

(1) Set the motor power and pole pair number, and determine the inverter operating current.

(2) Set the maximum output frequency and base frequency of the inverter and set the motor torque characteristics. The selection of voltage/frequency (v/f) working mode includes items such as maximum operating frequency, basic operating frequency (i.e. base frequency) and torque type.

The maximum frequency refers to the maximum frequency at which the inverter motor system can operate. Since the inverter's own maximum operating frequency may be relatively high, when the motor's maximum allowable operating frequency is lower than the inverter's maximum operating frequency, the operating frequency should be set according to the requirements of the motor and its load.

The basic operating frequency is the dividing line between the inverter's constant power control and constant torque control of the motor, and should be set according to the rated voltage of the motor.

Torque type refers to whether the load is a constant torque load or a variable torque load. The user selects one of the working modes according to the V/F working mode diagram and load characteristics in the inverter manual. General inverters are equipped with multiple V/F curves for users to choose from. When using, the appropriate V/F curve should be selected according to the nature of the load. If it is a fan or pump load, the torque operation code of the inverter should be set to variable torque and reduced torque operation characteristics. In order to improve the low-speed performance of the inverter when starting, so that the output torque of the motor can meet the starting requirements of the production load, the starting torque should be adjusted. In the three-phase asynchronous motor variable frequency speed regulation system, the control of torque is relatively complex. In the low frequency band, since the influence of resistance and leakage reactance cannot be ignored, if V/F is still kept constant, the magnetic flux will decrease, thereby reducing the output torque of the motor. Therefore, in the low frequency band, the voltage should be appropriately compensated to increase the torque. Generally, the inverter is manually set by the user for compensation.

(3) Set the inverter to built-in keyboard operation mode, press the run key and the stop key respectively, and observe whether the motor can start and stop normally.

(4) Set the electronic thermal relay function according to the inverter instruction manual.

The threshold value of the electronic thermal relay is defined as the ratio of the rated current of the motor and the inverter, usually expressed as a percentage. When the inverter output current exceeds its allowable current, the overcurrent protection facility will cut off the inverter output. Therefore, the maximum threshold value of the inverter electronic thermal relay does not exceed the maximum allowable output current of the inverter. The inverter overload protection setting value can be modified.

2.4 Inverter load operation debugging

The following five steps must be performed for inverter load operation debugging (i.e. the inverter is connected to the motor and the motor is loaded).

(1) Manually operate the run stop button on the inverter panel and observe the motor run stop process and the inverter display window to see if there is any abnormality.

(2) If the inverter overcurrent protection action occurs during the process of starting/stopping the motor, the acceleration/deceleration time should be reset. The acceleration of the motor during acceleration and deceleration depends on the acceleration torque, and the frequency change rate of the inverter during the start/brake process is set by the user. If the motor's rotational inertia or load changes and the speed increases or decreases according to the preset frequency change rate, the acceleration torque may be insufficient, causing the motor to stall, that is, the motor speed is not coordinated with the inverter output frequency, causing overcurrent or overvoltage. Therefore, it is necessary to reasonably set the acceleration and deceleration time according to the motor's rotational inertia and load so that the frequency change rate of the inverter can be coordinated with the motor speed change rate. The method to debug whether this setting is reasonable is to first select the acceleration and deceleration time according to experience and set it. If overcurrent occurs during the starting process, the acceleration time can be appropriately extended; if overcurrent occurs during the braking process, the deceleration time can be appropriately extended.

(3) If the inverter still has overcurrent protection action within the specified time, the start/stop operation curve should be changed, such as from a straight line to an S-shaped, U-shaped line or an inverse S-shaped, inverse U-shaped line. When the motor load inertia is large, a longer start/stop time should be used, and the operation curve type should be set according to its load characteristics.

(4) If the inverter still has overcurrent protection action, you should try to increase the maximum current protection value, but you cannot cancel the protection, and you should leave at least 10% to 20% protection margin. If this action still occurs, you should replace the inverter with a larger power level.

(5) If the inverter fails to drive the motor to reach the preset speed during the starting process, there may be two reasons:

The electromechanical resonance of the system can be judged from the sound of the motor running. The resonance point can be avoided by setting the frequency jump value (generally, the inverter can set three jump points). When the inverter with v/f control mode drives the three-phase asynchronous motor, the current and speed of the motor will oscillate in certain frequency bands. In severe cases, the system cannot run, and even overcurrent protection occurs during acceleration, making it impossible for the motor to start normally. It is more serious when the motor is lightly loaded or has a small moment of inertia. Ordinary inverters are equipped with a frequency jump function. Users can set the jump point and jump width on the v/f curve according to the frequency point where the system oscillates. When the motor accelerates, these frequency bands can be automatically skipped to ensure the normal operation of the system.

The torque output capacity of the motor is not enough. The factory parameter settings of inverters of different brands are different, and the load capacity is different under the same conditions. The load capacity of the motor may also be different due to different inverter control methods; or due to different system output efficiency; causing the load capacity to be different. In this case, the value of the torque boost can be increased. If it cannot be achieved, the manual torque boost function can be used, but do not set it too large, otherwise the temperature rise of the motor will increase. If it still does not work, a new control method should be used.

2.5 System debugging of inverter connected to host computer

After the manual basic setting debugging is completed, if there is a host computer in the system, connect the control line of the inverter directly to the control line of the host computer, and change the operation mode of the inverter to terminal control. According to the needs of the host computer system, adjust the range of the inverter receiving frequency signal terminal to 0~5v or 0~10v, and the response speed of the inverter to the analog frequency signal sampling.

2.6 Selection and trial of inverter control method

Here we only discuss two typical control methods commonly used by frequency converters, namely flux vector control and voltage/frequency (i.e. v/f) control.

(1) Inverter Flux Vector Control Method

The three-phase asynchronous motor and DC motor have the same torque generation mechanism in theory. Based on the principle that the product of magnetic field and the current perpendicular to it is equal to torque, the stator current supplied to the motor is divided into two parts, namely the magnetic field current that generates magnetic field and the torque current that generates torque. The vector control method is to decompose the stator current into magnetic field current and torque current, control them separately, and supply the synthesized current of the two to the motor at the same time, so as to obtain the same control characteristics as the DC motor. This control method can provide sufficient starting torque and sufficient low-speed torque, which is particularly suitable for occasions with large load changes. However, its operating conditions have the following four limitations:

The inverter capacity margin is generally required to be one level larger than the motor capacity;

There are more requirements for the number of motor stages;

Can only be used for single machine operation;

The length of the motor power cord cannot be too long.

When the above conditions cannot be met, a series of problems such as insufficient torque or motor speed fluctuation will occur. Therefore, unless the load changes greatly, it is recommended to use the v/f control method.

(2) Voltage/frequency (v/f) control method of frequency converter

The so-called voltage/frequency (v/f), that is, voltage/frequency, refers to the ratio of the output voltage and output frequency of the inverter within the control variable range to be coordinated. The base frequency is the dividing point between the constant torque characteristic operation and the constant power characteristic operation of the motor. Therefore, the voltage/frequency control mode and characteristics can be set according to the load's requirements for torque and power to achieve the desired control purpose.

3. Things to note when debugging

When debugging the inverter on site, pay attention to the following matters:

(1) Pay attention to the setting of the output frequency range

The setting of the inverter output frequency range, that is, the upper and lower limit values ​​of the inverter output frequency, is set to prevent the output frequency from being too high or too low due to misoperation or failure of the external frequency setting signal source, so as to prevent damage to mechanical equipment. This setting is generally set based on the maximum speed of the controlled motor or the empirical value.

(2) Pay attention to the setting of acceleration time

The acceleration time is the time required for the output frequency to rise from zero to the maximum frequency, and the deceleration time is the time required for the maximum frequency to fall to zero. The rise and fall of the common frequency setting signal are used to determine the acceleration and deceleration time. When the motor is accelerating, the given frequency rise rate is limited to prevent overcurrent, and the drop rate is set to prevent overvoltage when decelerating. When setting the acceleration time, it is also necessary to limit the acceleration current to below the overcurrent capacity of the inverter to prevent the inverter from tripping due to overcurrent; when setting the deceleration time, it is also necessary to prevent the voltage of the smoothing circuit from being too large, so as not to cause the inverter to trip due to regenerative overvoltage stall. The acceleration and deceleration time can be calculated according to the load, but it is relatively cumbersome. The simple method is: according to the load size, set a longer acceleration and deceleration time based on experience, and then observe whether there is an overcurrent or overvoltage alarm through operation, and gradually shorten the set time. The principle is that no alarm is issued during operation, and repeat the operation to determine the best value.

(3) Pay attention to the setting of motor protection function

In actual projects, the rated current of the motor should be used as the set value, which is the reference value for motor overload. However, it should be noted that this function setting is invalid when one inverter is used to control multiple motors.

(4) Pay attention to the selection of IGBT-related fast fuses

IGBT is the most important device in the inverter. It is a high-power field effect compound tube. The manufacturer uses semiconductor fast fuse to protect it. Its fusing time is shorter than the breakdown time of IGBT. If its performance changes, it will burn out the IGBT, so the selection of fast fuse model is very important.

(5) Pay attention to the setting of the number of automatic fault resets and the reset time

This setting is very important. In actual operation, occasional faults are inevitable, but they can be automatically overcome in an instant, ensuring the smooth operation of the inverter without having to find the fault point.

4 Conclusion

This article briefly discusses the specific steps, methods, techniques, key points and precautions for on-site debugging of frequency converters. Although there are many varieties of frequency converters in practice, and the debugging problems that may be encountered are also numerous, the debugging principles and related technical key points of frequency converters are similar, which is the intention of this article. (end)